The crew discover a huge, damaged pod ship of unknown alien origin. Uhura discovers that this vessel is the source of the radio emissions. Kirk orders Sulu to maneuver the Enterprise alongside the ancient starship, and then he asks Spock for readings on the ship. Spock reports that the temperature is absolute zero and that there is no energy or thermal reading in the ship to support life or send the radio messages. The only thing detected is a magnetic reading that could be normal for the metal used in the ship's construction. To Kirk's questions as to whether the metal or design can be identified, Spock replies negatively, saying both aspects of the craft are unidentified. However, Spock ascertains the ship as having been in the same orbit it currently is in for over 300 million years.

The landing party on board the insectoid vessel

Kirk decides to board the impressively designed starship with a landing party consisting of Spock, Scotty, and Dr.McCoy, so they strap on their life support belts and beam over. The ship appears to have been built by an insectoid race; hexagonal windows aboard the vessel, however, look precisely like the individual cells in honeycombs of bees. However, the craft's metal, rather than having been cast or rolled, has remarkably been drawn into filaments and spun, which a fascinated Kirk believes is much like a spider spins its web. The metal is also, as Spock points, lighter and stronger than any material currently available to the Federation. The landing party then notices that every pod on the ship has been burst open, apparently from the inside. Although McCoy conjectures the widespread breach of the pods "must have been" some kind of accident, Spock refutes this theory, stating that the systematic nature of the breakage suggests that the vessel's crew themselves broke the pods and essentially destroyed the craft.

"Captain's Log: We have beamed aboard the alien ship found orbiting a strange dead star. The Enterprise is recording all data for the log and a full report later."

Kirk contacts the Enterprise and reaches Lieutenant Uhura. Upon him asking, she notifies Kirk that she is no longer receiving the radio signal from the alien ship, the transmission having ceased when the landing party beamed aboard. Kirk then requests she keep a transporter lock on them because he has decided to investigate the inside of the strange spaceship. Spock registers a slight energy reading on his tricorder. He tells Kirk that it seems the ship is receiving the energy being generated by the away team and that the craft is apparently storing up the energy. The whole ship was made for just that purpose.

When the Enterprise crew enter the control center of the ship, the door seals behind them and an interference field prevents the landing party's communicators and phasers from operating. The source of this field turns out to be a device attached to the navigational console. Spock describes it as "not part of the normal equipment; it's like something they jury-rigged during an emergency." The field's purpose seems to have been to shield the control center from an intruder.

Further investigation is interrupted when something begins trying to break through the door of the control center. A log entry begins playing, explaining that the original crew chose to destroy their vessel to avoid carrying a malevolent entity to other worlds. As the door collapses and explosions begin to destroy the control center, the interference field fails and the landing party is beamed back aboard the Enterprise by Kyle.

Questar M-17 inhabited by an unidentified magnetic organism

After materializing in the transporter room, they discover that the malevolent entity has beamed aboard with them. It takes over control of the ship's systems, using the Enterprise's phasers to destroy the insectoid craft. The entity wants to use the Enterprise to take it away from the dead star and takes control of the bridge's automatic defense system. But Spock has placed the navigation console inside a static shield, so the entity cannot steer the ship. However, the entity has taken the crew hostage. Kirk acts as if he is obeying the entity, but actually plans to use the slingshot effect to break the Enterprise out of orbit. The entity, thinking Kirk is going to crash the Enterprise into the star, flees for its life while the ship frees itself from Questar's gravitational pull, leaving the creature to orbit around the star forever, wailing to the crew in terrible, endless loneliness.

"Look. Every pod...they've all been burst open."
"Aye, from the inside from the looks of them."
"Must have been some accident to get almost every pod."
"Accidents seldom have such system, Dr. McCoy. I believe we must consider the alternative possibility that the crew of this ship destroyed her... themselves."

- Kirk, Scott, McCoy, and Spock

"The whole ship is designed to receive and store energy."

- Scott

"A physiological symptom of latent primal superstition. The fear of primitive people confronting something unknown to them."
"Compared to the people who built this ship, we are primitives. Even you, Mr. Spock."

- Spock and Kirk

"Danger! Danger! The dead star... we are being drawn to it! Rather than carry this malevolent life form to other worlds, we have decided to destroy our own ship! There is no other answer! If you understand this message, you are protected only for this moment in this room! This thing, it wants..."

- Insectoid captain

"Jim, you don't think that's going to help us. Whatever that thing is, it survived a millennia in a dead hulk. All it has to do here is outlast us and just take over."
"No. It must be held by the magnetic force of the dead star. And it needs a starship to break free and a crew to man it."
"You are correct, Captain James T. Kirk. And I have the starship I've waited for so long, so terribly long!"

- McCoy, Kirk, and magnetic organism

"You will now remove the static shield from the navigation console, Captain James T. Kirk."
"You have shut down life support systems and endangered members of my crew. Restore those systems first!"
"All non-essential systems are extinguished. You will obey me."
"And if I refuse?"
"Obey me!"

"What are we dealing with, Spock?"
"High rank probabilities, captain. It is a magnetic organism without mass, but capable of symbiotic relationship with a host body; a starship, for instance. It is a form of primal energy, and it can utilize the electronic control systems of a starship like the mind of a man uses the neural control systems of the Human body. It has become the Enterprise, and we are only life-support organisms in its body like the white corpuscles in Human blood. And captain, the magnetic flux readings are higher. It is growing stronger, building itself.

- Kirk and Spock, on the magnetic organism

"You will leave this orbit and plot course to galactic coordinates zero-three-six-point-two-three-one!"
"That's the heart of the galaxy, captain!"
"Plot the course, Mr. Sulu."
"Captain, this symbiote can reproduce itself by mitosis and take over every starship we encounter. It can control computer centers... whole planets."
"I'm aware of that, Mr. Spock."
"Complete repairs! Obey me!"

- Magnetic organism, Sulu, Kirk, and Spock

"No! Accelerate! Do not destroy the ship! Obey! Obey!! OBEY!!!"

- Magnetic organism

"Is it gone?"
"Affirmative. It fled the ship when it thought we would crash into the dead star."
"Don't leave me alone! Please! Please! So lonely...!"

Lieutenant Kyle reappears from the original series here, though with a mustache, brown hair, and voiced by James Doohan, replacing John Winston. This is the only TAS episode in which Kyle has any dialogue.

When the cutter beams cut the hinge of the core hatch trapped on Scott, the sound effect it makes when it hits the floor is a common sound effect from The Flintstones (a loud clang noise).(citation needed • edit)

Gene Roddenberry once related that the medium of animation made it easy to depict a massive starship such as the one featured here. "If we want an exotic space ship fifty miles across," he said, "it's as easy to draw that as it is to do one the size of the Enterprise." [1] According to background artist Robert Kline, though, pleasing Roddenberry with a feasible, unseen design for the insectoid ship herein was the hardest challenge in the creation of Star Trek: The Animated Series and required "literally 100 tries." (Gene Roddenberry's Star Trek: The Original Cast Adventures, p. 156)

The cutting beam in this episode emanates from an instrument that looks almost exactly like the hand-held spectrum analyzer used by Spock in the TOS episode "The Naked Time".

This episode marks the first on-screen appearance of an entirely non-Human member of Starfleet, though the USS Intrepid was stated to have an all-Vulcan crew in the TOS episode "The Immunity Syndrome".

Like in "Charlie X", the crew abandon a lonely individual who has caused trouble for them.

On 4 June 1973, NBC made the announcement that Star Trek's regular cast had reunited to record the script for this installment (as well as the teleplays for an additional two episodes). (Star Trek: Communicator issue 119, p. 32)

At one point during the first week of September 1973, this episode was shown at a private NBC screening, at which Los Angeles Times critic Cecil Smith was overheard remarking, "This is definitely not a kid's program." Smith's positive impression of the episode influenced a review (entitled "Star Trek Bows in Animated Form") that was written by him and was published in the 10 September 1973 edition of the Los Angeles Times. (The Making of Star Trek: The Motion Picture, pp. 19 & iv)

Andy Mangels stated, "When excited viewers sat down in front of their television sets on September 8, 1973, they weren't certain what to expect, but they knew one thing: Star Trek was back on the air! [....] It was an auspicious beginning that promised viewers a return to the 'five year mission' they knew and loved." [2](X)

On its initial broadcast in September 1973, this episode faced tough competition. The Monster Times stated it "was completely decimated" in the ratings by other programs airing on the same morning. (Star Trek Magazine issue 180, p. 64)

The editors of Trek magazine collectively scored this episode 1 out of 5 stars (a rating that they termed "poor"). (The Best of Trek #1, p. 111)

In the unofficial reference bookTrek Navigator: The Ultimate Guide to the Entire Trek Saga (p. 21), co-writer Mark A. Altman rates this episode 2 out of 4 stars (defined as "mediocre") while fellow co-writer Edward Gross ranks the episode 2 and a half out of 4 stars (defined as "average"). Altman describes the episode as "an unspectacular entry" with an "unremarkable" story and, despite recognizing "some nice touches" (counting the life-support belts among them), he laments a lack of "new" elements in the installment, finding that aliens hijacking the Enterprise has been done too much and that the episode's only innovative aspect is the design of the pod ship. Altman concludes, "It's enough to leave one longing for Sybok and Star Trek V: The Final Frontier." Gross agrees that much about the episode has been seen before but also comments, "Samuel Peeples [...] has taken the animated format and attempted to concoct a script whose sheer scale is unlike anything that could have been accomplished in the live-action show back in the 1960s." Gross also describes the alien's loneliness-fearing pleas at the end of the episode to be "a bit touching" but ultimately, considering the entity's other actions, "too little too late."

In the "Ultimate Guide" published in Star Trek Magazine issue 163, p. 24, this episode was rated 4 out of 5 Starfleet arrowhead insignia and was regarded as the fourth best installment of the animated Star Trek. The magazine also commented, "The scale and design of the ship perfectly exploits the medium. A classic Kirk bluff to defeat an energy being that takes over the Enterprise is a perfect cap to the episode."

A limited-edition collector's animation cel inspired by this episode was once available from Filmation. (A Trekker's Guide to Collectibles, p. 46) The cel was number "ST-16" and illustrated the Enterprise drifting in between the massive pod ship. [5]

A Daren Dochterman illustration of the pod ship is featured in the Ships of the Line book (pp. 70-71), a publication that is made up of images from the related calendars.